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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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26th Mar 2018, 5:39 pm | #1 |
Heptode
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Question about flyback diode
My question is simple and I hope the answer to be simple too
Is it possible to use a flyback diode as a rectifier to obtain 1500V DC? From the info on the datsheets of such diodes I'm not sure what to think. My options are: 6D20P, PY81, and PY801 Thanks in advance
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26th Mar 2018, 7:40 pm | #2 |
Dekatron
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Re: Question about flyback diode
That's a booster diode rather than an EHT rectifier diode (which rectifies the flyback)?
I'd go with Yes, though voltage is getting a bit high. You don't say what waveform is being rectified, but if it's a sinewave, the diode will see 3kV reverse across it. |
26th Mar 2018, 8:11 pm | #3 | ||
Heptode
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Re: Question about flyback diode
Quote:
Quote:
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26th Mar 2018, 9:20 pm | #4 |
Dekatron
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Re: Question about flyback diode
Doesn't the booster diode rectify the the raw flyback voltage from the LOPT primary whereas the EHT rectifier works on stepped-up-by-the-overwind flyback voltage?
Both of them handily use energy which would otherwise have to be dissipated in the line scan circuits.
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26th Mar 2018, 10:22 pm | #5 |
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Re: Question about flyback diode
The Booster diode returns stored magnetic energy of the yoke and lopty (present at the end of line scan) via damped current back to the power supply over the first 1/3 or so of the next scan line) This lowers the average power consumption of a line output stage which it is why its also called an efficiency diode and it damp oscillations too, which is why its called a damper diode and it generates a DC level across the B+ boost capacitor which is why its called a booster diode at times, so the many names don't help.
The EHT diode (possibly sometimes called a flyback diode but better called the EHT rectifier) is a half wave rectifier with rectifies the flyback voltage peak outside of scan time. But the terminolgy is loose and I have seen booster diodes referred to as flyback diodes. But to clarify: EHT rectifiers have very high voltage but low current capability. Booster diodes/rectifiers have fairly high current and fairly high voltage ratings. Due to the fact that in valve sets the peak voltages are high, booster diodes as suggested by the OP can have applications in high voltage medium current power supplies. A PY81 for example has a max PIV of 4.5kV, but specified for 18% duty cycle, so for a 50% cycle probably best to de-rate that to maybe 3.5kV and an average anode current of 150mA. These valves also have amazing heater cathode insulation, but better to connect the heater to the cathode if they are used in an HV power supply. Unfortunately many spec sheets for booster diodes don't give a max power rating and you just deduce it from the max average anode current and voltage, but as I mentioned that is often specified for a low duty cycle, so its hard to know exactly. Last edited by Argus25; 26th Mar 2018 at 10:29 pm. Reason: typo |
27th Mar 2018, 12:44 am | #6 |
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Re: Question about flyback diode
PS:
Some years ago I noticed that the topic of efficiency diodes was not well covered in a lot of TV texts, but more interestingly how they came about or evolved. It was Blumlein that proposed the idea, but it was RCA labs that perfected the explanation of the Physics of it and commercialized it. I have a copy of RCA original research papers by Otto Schade on the issue of energy recovery scanning, which I have been meaning to scan and upload to my website. It appeared in their first Post war set, the RCA621TS in 1946, but it was actually designed in the early 1940's. Here is an article I did on the topic for those interested in how design iterations caused the "modern" valve line output stage configuration to come about: http://worldphaco.com/uploads/TELEVI...R_DIODE_OR.pdf Certainly with booster diodes, planned to be used in say a mains based power supply instead, it would be good to know the voltage drop across the type at any current, to calculate the dissipation, and know the max dissipation for the type, but its often not noted. Last edited by Argus25; 27th Mar 2018 at 12:52 am. |
28th Mar 2018, 1:25 pm | #7 |
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
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Re: Question about flyback diode
Thank you very much for your interesting answers.
My intention is to use a vacuum diode to supply the -1,5kV of a Soviet CRT, that's why I'm asking. Current or power would not be a problem as it just require less than 1 mA. I'm concern about reverse voltage as it would be 3kV as Kalee20 said, and in the datasheets one can only find peak values but not repetitive 50% duty cycle value (I know, these diodes are not designed for PSU) I just wanted to know if somebody have used them in this role with such a voltage. I have silicon diodes capable to stand such voltage but I'm after an all-vacuum unit. I have an 1B3 diode which I intend to use for the +4kV required by the CRT
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28th Mar 2018, 10:12 pm | #8 |
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Re: Question about flyback diode
I think the plan to use the booster diode would be fine, the current is so low.
One thing though, most likely, for powering the CRT with +4.5kv and -1.5kv; it is just the circuit design that requires that, not the CRT itself. So you could probably just reconfigure the design to a single +6kV supply, paying attention to the voltage ratings of coupling capacitors to the CRT's deflection plates (assuming its electrostatic) and any grid & cathode coupling caps. One reason split supplies were used for CRT's is that lower voltage capacitors could be used and the insulation requirements of the wiring and EHT rectifier socket etc were a bit less. Some designs even grounded the final anode and deflection plates and had the grid/cathode at a high negative potential to make coupling into the deflection plates easier without high voltage rated capacitors. But high voltage rated caps are easy to get and these days quite compact. If you posted the circuit I could advise, probably, how to configure it for a single supply. |
28th Mar 2018, 11:21 pm | #9 |
Dekatron
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Re: Question about flyback diode
If the PIV is rated at 4.5kV, then I'd be pretty confident about running at 3kV repetitively, which is what you'd get with half-wave rectifying a 1.5kV peak sinewave supply to generate 1.5kV DC.
I've used a PY81 as a rectifier in a flyback switch-mode power supply at 30kHz, although it was only generating 300V. It might be worth considering what happens if the valve flashes over. In the absence of anything else, a couple of inches of 40SWG wire in a tube, in series, might make a crude HV fuse. No doubt someone will comment on this - I don't have much experience above 1kV! |
29th Mar 2018, 1:27 pm | #10 | ||
Heptode
Join Date: Dec 2011
Location: Mayabeque, Cuba
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Re: Question about flyback diode
Quote:
On the other hand, the deflection plates won't be connected via capacitors as I want it to measure DC signals too, so the voltage applied to them can not be so high. Quote:
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29th Mar 2018, 11:17 pm | #11 |
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Re: Question about flyback diode
It looks like, with that particular type of CRT, with the low range voltages in the region of the deflection plates, it would be best to stick to their original design. Many other types of CRT run the deflection plates at a similar potential to the final anode or just below, but not this one it appears. It was likely for a scope with direct coupling to the deflection plates at around HT potentials and it therefore has post deflection acceleration with the +3.5kV final anode.
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30th Mar 2018, 1:18 am | #12 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
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Re: Question about flyback diode
Similar to making vintage amplifier power supplies more "bullet proof", and with less collateral damage should a valve diode go gassy or otherwise fail, you could surreptitiously insert an ss diode (eg. voltage balanced string of 1kV diodes) in series with the valve diode anode.
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